Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad-hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. Some of these systems may operate in an unlicensed spectrum.
Recently, device-to-device (D2D) communications, as an underlay to cellular networks have been proposed to take advantage of the physical proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment to provide proximity-based services (ProSe). It is suggested that such device-to-device communication shares the same spectrum as the cellular system, by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative since spectrum is a scarce resource and dynamic sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency. The transmission mode when sending data during D2D communication may be either unicast with a specific user equipment (UE) as the receiver; multicast (e.g., group-cast) with a group of UEs as receivers; or broadcast with all UEs as receivers.
With connectionless D2D communication, data can be sent from one device to another device without prior arrangement, thereby reducing the overhead and increasing the communication capacity of the cellular system which is crucial in emergency situations. The source device, a UE, communicates data to one (unicast) or more devices (multicast/group-cast/broadcast), without first ensuring that the recipients are available and ready to receive the data. Connectionless communications may be used for one-to-one or one-to-many communications, but it is particularly effective for multicast and broadcast transmissions and, thus, well-suited for broadcast and group communication. The connectionless communication may be realized, for example, via PHY unicast, multicast, group-cast, and/or broadcast transmissions. With PHY broadcast transmissions, the transmissions may still be turned into unicast, group-cast, or multicast at higher layers. For example, in the MAC layer, multicast or even unicast addresses may be used. Alternatively, if using broadcast on both PHY and MAC, multicast or unicast IP addresses may be used at the IP layer.
One of the ways to support efficiently D2D communication is to use a scheduling assignment (SA). FIG. 2 illustrates the data transmission procedure from UE-A to UE-B that are both outside network coverage at the time of data transmission. As a prerequisite, UE-A and UE-B are preconfigured with, for example, resource pool information (time and frequency configuration) used later for data transmission. When UE-A needs to communicate data to UE-B, it sends a sync signal, which later is used as a time reference by UE-B. The next step is to communicate a scheduling assignment, followed by the actual data.
Scheduling assignments (SA) are control messages used for direct scheduling of D2D communications. SAs are communicated by the UE that intends to transmit D2D data and are received by the UEs that are potentially interested in the D2D data. The SAs are communicated on dedicated resources characterized by time and frequency, and is typically a sparse resource. SAs provide useful information that can be used by the receiver to correctly decode the D2D data transmission associated to the SA. For example, the useful information can be the resources for data transmission, the modulation/coding parameters, timing information, identities for the transmitter and/or receiver, etc. SAs may be communicated prior to the actual data transmission, so that a receiver is able to selectively receive data based on the content of the SAs. These data transmissions scheduled by a SA may be known as a “transmission pattern”.
In FIG. 3, an example is shown of the time and frequency resources used to schedule assignments and data. It is assumed that the frequency resource for data is the same as for the SA. The time resource for data is provided by the time-pattern information element in the SA itself.
By monitoring the identities carried in the scheduling assignments (SA), discontinuous reception (DRX) is enabled in the UE. For example, for multicast D2D communication, the identity in the SA is derived from the identifier of the multicast group, such as the eight least significant bits of what is known as the Layer-2 Group ID. Thus a UE which is interested in receiving data from one or several multicast groups, needs to check the SAs for the corresponding identities. When the receiving UE receives an SA, with an identity which corresponds to one of the multicast groups that the UE is interested in, the receiving UE may decode the data pointed out by the other information carried in the SA.
A method exists for providing redundancy in the scheduling assignments. For example, the same content is communicated by multiple SAs, and the transmitter may communicate only a subset of the redundant SAs, based on autonomous decisions or predefined patterns. As such, this allows the transmitter to periodically monitor the SA resources.
Thus, the scheduling assignments and communication data, for D2D, may communicate on a scarce resource pool associated with time and/or frequency within a spectrum. The UE may select the resource(s) to communicate the SAs and data by using some rules or by trying to find a free resource based on previous SA cycles. However, in cases of high amounts of traffic within a network, these resource pools may become full and thus, there is no way to prioritize between different UEs. Moreover, there is no way for one UE to claim a SA and/or data resource in case of this UE having “higher priority” than other UEs.